The present invention relates generally to gas turbine engines, and, more specifically, to impingement cooling therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor and ignited for generating hot combustion gases which flow through one or more turbine stages. The combustor and various components downstream therefrom are therefore subject to heating from the hot combustion gases and are typically cooled for obtaining a suitable useful life thereof.
The combustor itself is typically cooled by providing film cooling holes through which a portion of compressor discharge air is channeled for developing a boundary layer or film of cooling air along the inner surface of a combustor liner which protects the liner from the hot combustion gases and provides cooling thereof.
Disposed immediately downstream from the combustor is a high pressure turbine nozzle having a row of hollow turbine vanes through which another portion of the compressor air is channeled for providing internal cooling thereof.
In both of these examples, enhanced cooling may be provided using an impingement baffle having a plurality of laterally spaced apart impingement holes through which respective portions of the cooling air is directed in jets against the outboard surfaces of the combustor liner or against the inboard surfaces of the turbine vanes. Impingement air provides local cooling of the corresponding hot walls subject to the combustion gases on the opposite sides thereof, and is typically used in conjunction with other forms of cooling such as film cooling. The spent impingement air may therefore be used initially for impingement cooling and then may be discharged through the hot wall as film cooling air for performing double duty and maximizing the cooling effectiveness thereof.
Impingement baffles require close spacing with the hot wall through which the spent impingement air is channeled after impinging the wall. The spacing is maintained by using spacers in the form of ribs or solid tabs which extend between the baffle and the hot wall. The tabs must be integrally formed with at least one of the two components, and are therefore subject to local variation in temperature distribution due to heat transfer between the hot wall and the relatively cool impingement baffle. Local temperature variation results in local differences in thermal stress which must be minimized for obtaining a useful life.
Crossflow of the spent impingement air channeled between the baffle and the hot wall degrades the effectiveness of downstream impingement jets by deflection thereof from the preferred normal impingement. The spacer tabs may be used to advantage by being configured and spaced between adjacent impingement holes for providing local shields to protect downstream impingement jets from the crossflow resulting from impingement jets upstream therefrom. However, solid integral spacer tabs introduce relatively large thermal stress due to associated temperature gradients between the cool baffle and the hot wall which can reduce the useful life thereof.
Furthermore, the individual spacer tabs require precise placement adjacent to the impingement holes for being effective, with misalignment thereof degrading the shielding effectiveness. Yet further, integral spacer tabs limit the ability to introduce inclined film cooling holes through the hot wall which are typically made therein by drilling, using lasers for example. Since it is impermissible to damage a spacer tab when laser drilling the film cooling holes, the holes must be maintained at a suitable distance from respective ones of the tabs to prevent damage thereto. This limits the area in which the film cooling holes may be placed and thusly affects the cooling effectiveness thereof and the corresponding life of the parts.
Accordingly, it is desired to provide an improved impingement baffle which is readily manufactured, and has reduced thermal stress during operation and does not interfere with the formation of the film cooling holes for example.